Fill a scuba tank up to 10psig (sea level, outside on a boat) and put that scuba tank into a decompression chamber and pressureize the decompression chamber to 24.7psia.  If you were living inside the decompression chamber with the scuba tank that was filled with 10psig, and you put a pressure gauage (psig) on the tank.  What would that psig read that you just put on the scuba tank?  Would it be 0 because the ambient air pressure (decompression chamber 24.7psia) is equal to the scuba tank pressure (10psig)?

In most common pressure gauges there is a bent tube connected to the needle on the dial.  Internal pressure tries to straighten the tube out.  The amount that it succeeds is the reading on the pressure gauge.  Atmospheric air is on the outside of the tube and is resisting the movement.  The gauge naturally reads the difference between the pressure in the tube and atmospheric pressure.

I think you should have your bar buddies put their extensive middle-school educations to the test and try it.  Most dive schools have a decompression chamber.  

The barometric chamber simulates atmospheric pressure so if you put your scuba tank with 10 psig into the chamber and pressured the chamber up to 23.1 ft of water or 14939 ft below sea level (24.7 psia), the gauge on the tank would read zero and if you open the valve/Balance Valves you would not get any organized flow (there would be some gas exchange, but the flow would be equally as likely to be predominantly in as out if the gauges were both well calibrated).

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